The present invention relates to electric transformers, especially distribution transformers and to protective equipment therefor.
Investigations have been made in recent years of failures of common designs of single-phase overhead and padmounted distribution transformers caused by lightning current surges. As a result of these investigations, it has been found that anomalous failures of such transformers are attributable to lightning surges entering the transformers via the normally unprotected low voltage terminals, causing failure of the high voltage winding. By "anomalous failures" is meant failures that occur despite state-of-the-art lightning protection. Thus, it was found that high voltage arrester protection of the high voltage coil was ineffective in avoiding the problem.
Surge currents can enter the low-voltage terminals of an overhead distribution transformer in three basic ways. The first, and most obvious way, is due to direct lightning strikes on secondary service conductors. In this case, surge currents are forced through the transformer secondary windings on their way to ground at the transformer neutral. This mode of current surge may involve only one half, or the entire secondary winding.
A second possible mode of surge current entry, into the low voltage windings of a distribution transformer, is due to lightning discharge into the ground near a secondary service point. Such a discharge can cause a local elevation of ground potential resulting in ground currents flowing outward from the discharge point. Under this condition, it is possible for current to flow from the elevated potential customer ground toward the distribution transformer. Some of this current can flow through the transformer secondary windings resulting in low-side current surge. This mode is possible in both overhead and pad mounted transformers.
The third way that current surges enter low-voltage windings may be less obvious than the others, but is perhaps the most common in occurrence. Lightning strikes to overhead conductors are shunted to ground at the service pole by a ground wire running down the pole. The lightning arrester connected to the distribution transformer shunts part of the surge from the phase conductor, while the overhead neutral conductor is connected directly to the pole ground. Since the transformer neutral is also connected to this ground wire, part of the discharge current can be diverted into the secondary circuit. This mode is also possible in both pole type and in pad mounted transformers.
In this last case, surge current enters the center terminal of the low-voltage winding and divides through the two halves of the winding, exiting by way of the two secondary line terminals. For current to flow through the transformer, there must be a path through the customer load or meter. The amount of surge current may be dependent upon the amount of customer load connected at the time of the surge. Also, if there is a poor or high-resistance pole ground, a greater percentage of surge current will be diverted to the transformer windings. However, if the pole ground is adequate, the current level within the transformer will be well below that required to produce an insulation failure within the windings.
Particularly affected are the following types of conditions:
Uncompensated winding constructions, i.e., non-interlaced low voltage windings. When 3-wire surge operation occurs, e.g., surge enters X2 and departs from transformer from both X1 and X3. PA1 Both compensated and uncompensated winding constructions, i.e., both interlaced and non-interlaced low voltage windings. When 2-wire surge operation occurs, e.g., surge enters X2 and departs from transformer only on X1.
The majority of modern day distribution transformers are constructed as shell form non-interlaced low voltage and are thus susceptible to 3-wire surge operation damage.
Virtually all modern day distribution transformers are susceptible to 2-wire surge operation damage. This can happen when only one meter gap fires or when the load on X1 is substantially different from that on X3.
Presently, used schemes for protecting against low side surges and their effectiveness are as follows:
______________________________________ Balanced Unbalanced 3-Wire Surge 2-Wire Surge ______________________________________ Interlaced low voltage Excellent No protection Extra Primary winding Fair. Raises Fair. Raises layer insulation damage damage threshold level. threshold level. Expensive Expensive Low voltage arrester Excellent but Excellent but using metal oxide expensive and expensive and varister technology non-visible non-visible ______________________________________